1) People will listen to rock anyway, might as well make good rock that is good for the soul. This is one of the driving reasons for Steve Rowe's ministry, to take away that stumbling block.
2) It has encouraged people. We are commanded to encourage each other, and music is an appropriate medium for that.
"Speak to one another with psalms, hymns and spiritual songs. Sing and make music in your heart to the Lord, always giving thanks to God the Father for everything, in the name of our Lord Jesus Christ." - Ephesians 5:19-20
"Let the word of Christ dwell in you richly as you teach and admonish one another with all wisdom, and as you sing psalms, hymns and spiritual songs with gratitude in your hearts to God." - Colossians 3:16
3) It has literally saved lives. Many teenagers think that killing themselves is an option, particularly if they grew up in abusive homes. Songs by P.O.D., KJ-52 and others have, confirmed by fan feedback, helped keep people from killing themselves. If you believe in free will, this wouldn't have happened any other way. If you don't, then God delighted to use this sort of music to accomplish His purpose. I don't know how those people are doing now, but they are certainly better off than they would have been had they succeeded in taking their own lives. Christian rock is a way that God uses to deliver people from Satan.
4) It gives positive role models. The world, they honor their musicians for taking more drugs, having more sex, spouting more blasphemy, and to be fair, also for musical talent. There are Christian bands out there who are great role models, however imperfect they may be (the same can be said of lots of other Christians). Have you ever had guns pulled on you specifically because you are a Christian? NIV has. Do you minister to people in the gutter of life? P.O.D. does.